JP7554770B2 - Method for producing modified cellulose microfibrils - Google Patents

Description

The present invention relates to a method for producing modified cellulose microfibrils.

Modified cellulose microfibrils are obtained by modifying cellulose raw materials such as pulp and then wet grinding them, and are expected to be used as an additive to improve the strength and water retention of paper (Patent Documents 1 and 2).

International Publication No. 2019/189588 International Publication No. 2019/189611

However, Patent Documents 1 and 2 do not describe the conditions for producing modified cellulose microfibrils. Modified cellulose nanofibers have a lower viscosity than nano-order cellulose nanofibers with an average fiber diameter of less than 500 nm, and are expected to be useful in a variety of applications, so there is a demand for the development of an efficient production method.

The present invention aims to provide a method for producing fibrillated modified cellulose fibers having a long average fiber length and a large aspect ratio.

The present invention provides the following [1] to [8].
[1] Step (1): A step of modifying a cellulosic raw material to obtain modified cellulose; and
Step (2): A step of mechanically treating the modified cellulose obtained in step (1) under conditions of a pH of 7 or less and a solid content concentration of 0.1 to 15% by mass to defibrate the cellulose;
A method for producing modified cellulose microfibrils, comprising:
[2] The method according to [1], wherein the modified cellulose is oxidized cellulose or etherified cellulose.
[3] The method according to [2], wherein the etherified cellulose is a carboxyalkylated cellulose.
[4] The method according to [2] or [3], wherein the carboxyl group content of the oxidized cellulose and the carboxyalkylated cellulose is 0.1 to 2.5 mmol/g.
[5] The method according to any one of [2] to [4], wherein the degree of carboxyl substitution of the oxidized cellulose is 0.01 to 0.50.
[6] The method according to [3] or [4], wherein the carboxyalkyl substitution degree of the carboxyalkylated cellulose is 0.01 to 0.50.
[7] The method according to any one of [1] to [6], wherein the mechanical treatment is a beating or defibration treatment.
[8] The method according to any one of [1] to [7], wherein the modified cellulose microfibrils have an average fiber diameter of 500 nm or more, an average fiber length of 500 μm or more, and an aspect ratio of 30 or more.

According to the method of the present invention, modified cellulose microfibrils having a long average fiber length and a large aspect ratio can be efficiently produced.

In the present invention, modified cellulose microfibrils can be produced by a method including the following steps (1) and (2). The order of steps (1) and (2) is not particularly limited, but steps (1) and (2) are preferably performed in that order.
Step (1): A step of modifying a cellulosic raw material to obtain modified cellulose; and Step (2): A step of mechanically treating the modified cellulose obtained in Step (1) at a pH of 7 or less and a solid content concentration of 0.1 to 15% by mass to defibrate the cellulose.

<Step (1): Modification step>
In step (1), a cellulosic raw material is modified to obtain modified cellulose.

(Cellulosic raw materials)
The cellulose-based raw material, which is the raw material of the modified cellulose microfibril, is not particularly limited as long as it is a pulp containing cellulose. Examples of the cellulose-based raw material include those derived from plants, animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), and microbial products. Examples of the cellulose-based raw material derived from plants include wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, and pulp (e.g., softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), softwood dissolving pulp, hardwood dissolving pulp, recycled pulp, and waste paper). The cellulose-based raw material may be any one of these, or a combination of two or more of them. Preferably, the cellulosic raw material is derived from a plant or a microorganism, more preferably, from a plant, and even more preferably, from pulp (eg, wood-based pulp).

The cellulose-based raw material preferably has a high cellulose I type crystallinity, more preferably 60% or more, and even more preferably 70% or more. This allows the cellulose I type crystallinity of the modified cellulose microfibrils to be maintained at 50% or more.

Cellulosic raw materials usually contain cellulose fibers. In this specification, cellulose fibers means fibrous cellulose before or after chemical modification, unless otherwise specified. Cellulosic raw materials may contain fiber components other than cellulose. The average fiber diameter of the cellulose fibers is not particularly limited, but examples are as follows. The average fiber diameter of the cellulose fibers of cellulosic raw materials derived from softwood kraft pulp is usually about 30 to 60 μm, and the average fiber diameter of the cellulose fibers of cellulosic raw materials derived from hardwood kraft pulp is usually about 10 to 30 μm. The average fiber diameter of the cellulose fibers of cellulosic raw materials derived from general refined pulps (other than softwood kraft pulp and hardwood kraft pulp) is usually about 50 μm.

(Chemical Modification)
In step (1), the cellulose-based raw material is subjected to a modification treatment (usually chemical modification), resulting in modified cellulose. In this specification, modification usually refers to chemical modification, and chemical modification means chemical modification, usually meaning chemical modification of the hydroxyl groups of the glucose units of cellulose. Cellulose is composed of glucose units and has three hydroxyl groups per glucose unit. Examples of chemical modifications include oxidation, etherification, esterification such as phosphoric acid esterification, silane coupling, fluorination, and cationization. Among these, oxidation (carboxylation), etherification (e.g., carboxyalkylation), cationization, and esterification are preferred, and oxidation (carboxylation) and carboxyalkylation are more preferred.

(Oxidation treatment (carboxylation treatment))
The oxidation treatment usually results in modification of at least one of the hydroxyl groups inherent to cellulose to a carboxyl group, and preferably at least one of the hydroxyl groups bonded to the carbon atom at the 6th position of the glucopyranose ring to a carboxyl group.

The oxidation method in the oxidation treatment is not particularly limited, but examples include a method in which the cellulosic raw material is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound and at least one of a bromide and an iodide. According to this method, a carbon atom having a primary hydroxyl group bonded to the carbon atom at the 6th position of the glucopyranose ring on the surface of the cellulose is selectively oxidized to generate a group selected from the group consisting of an aldehyde group, a carboxyl group, and a carboxylate group. The concentration of the cellulosic raw material during the reaction is preferably 5% by mass or less, but is not particularly limited.

An N-oxyl compound is a compound capable of generating a nitroxy radical. The use of an N-oxyl compound can promote the desired oxidation reaction. Examples of N-oxyl compounds include, but are not limited to, 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter also referred to as "TEMPO") and its derivatives (e.g., 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine-N-oxy radical: hereinafter also referred to as "4-hydroxyTEMPO").

The amount of N-oxyl compound used may be an amount that catalyzes the oxidation reaction of the raw material cellulose. For example, the amount is preferably 0.01 mmol or more, and more preferably 0.05 mmol or more, per 1 g of bone-dry cellulose. The upper limit is preferably 10 mmol or less, more preferably 1 mmol or less, and even more preferably 0.5 mmol or less. Therefore, the amount of N-oxyl compound used is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, and even more preferably 0.05 to 0.5 mmol, per 1 g of bone-dry cellulose. The amount of N-oxyl compound used in the reaction system is usually about 0.1 to 4 mmol/L.

Bromides are compounds containing bromine, such as alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, such as alkali metal iodides. The amount of bromide or iodide used is not particularly limited and can be selected within a range that can promote the oxidation reaction. The total amount of bromide and iodide is preferably 0.1 mmol or more per 1 g of bone dry cellulose, more preferably 0.5 mmol or more. The upper limit of this amount is preferably 100 mmol or less, more preferably 10 mmol or less, and even more preferably 5 mmol or less. Therefore, the total amount of bromide and iodide is preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol per 1 g of bone dry cellulose.

The oxidizing agent is not particularly limited, but examples thereof include halogens, hypohalous acids, hypohalous acids, perhalogen acids, their salts, halogen oxides, and peroxides. Among them, hypohalous acids or their salts are preferred because they are inexpensive and have a low environmental impact, hypochlorous acid or its salts are more preferred, and sodium hypochlorite is even more preferred. The amount of the oxidizing agent used is preferably 0.5 mmol or more per 1 g of bone-dry cellulose, more preferably 1 mmol or more, and even more preferably 3 mmol or more. The upper limit of the amount is preferably 500 mmol or less, more preferably 50 mmol or less, even more preferably 25 mmol or less, and even more preferably 10 mmol or less. Therefore, the amount of the oxidizing agent used is preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even more preferably 1 to 25 mmol, and even more preferably 3 to 10 mmol, per 1 g of bone-dry cellulose. When an N-oxyl compound is used, the amount of the oxidizing agent used is preferably 1 mol or more per 1 mol of the N-oxyl compound, and the upper limit is preferably 40 mol or less. Therefore, the amount of the oxidizing agent used per 1 mol of the N-oxyl compound is preferably 1 to 40 mol.

The conditions of pH, temperature, etc. during the oxidation reaction are not particularly limited. In general, the oxidation reaction proceeds efficiently even under relatively mild conditions. The reaction temperature is preferably 4°C or higher, more preferably 15°C or higher. The upper limit of the temperature is preferably 40°C or lower, more preferably 30°C or lower. Therefore, the reaction temperature is preferably 4 to 40°C, and may be about 15 to 30°C, that is, room temperature. The pH of the reaction solution is preferably 8 or higher, more preferably 10 or higher. The upper limit of the pH is preferably 12 or lower, more preferably 11 or lower. Therefore, the pH of the reaction solution is preferably 8 to 12, more preferably about 10 to 11. Usually, as the oxidation reaction proceeds, carboxyl groups are generated in the cellulose, so the pH of the reaction solution tends to decrease. Therefore, in order to proceed efficiently with the oxidation reaction, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution in the above range. The reaction medium during oxidation is preferably water for reasons such as ease of handling and the fact that side reactions are unlikely to occur. The reaction time in the oxidation can be appropriately set according to the degree of progress of the oxidation, and is usually 0.5 hours or more, with the upper limit usually being 6 hours or less, preferably 4 hours or less. Therefore, the reaction time in the oxidation is usually about 0.5 to 6 hours, preferably about 0.5 to 4 hours.

The oxidation may be carried out in two or more separate reaction stages. For example, the oxidized cellulose obtained by filtration after the completion of the first reaction stage can be oxidized again under the same or different reaction conditions, allowing efficient oxidation without reaction inhibition by table salt produced as a by-product in the first reaction stage.

Another example of oxidation is ozone oxidation. This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring that constitutes cellulose, and causes the decomposition of the cellulose chain.

The ozone treatment is usually carried out by contacting a gas containing ozone with the cellulosic raw material. The ozone concentration in the gas is preferably 50 g/m ³ or more. The upper limit is preferably 250 g/m ³ or less, more preferably 220 g/m ³ or less. Therefore, the ozone concentration in the gas is preferably 50 to 250 g/m ³ , more preferably 50 to 220 g/m ^3. The amount of ozone added is preferably 0.1 parts by mass or more, more preferably 5 parts by mass or more, per 100 parts by mass of the solid content of the cellulosic raw material. The upper limit of the amount of ozone added is usually 30 parts by mass or less. Therefore, the amount of ozone added is preferably 0.1 to 30 parts by mass, more preferably 5 to 30 parts by mass, per 100 parts by mass of the solid content of the cellulosic raw material. The ozone treatment temperature is usually 0° C. or more, preferably 20° C. or more, and the upper limit is usually 50° C. or less. Therefore, the ozone treatment temperature is preferably 0 to 50°C, and more preferably 20 to 50°C. The ozone treatment time is usually 1 minute or more, and preferably 30 minutes or more, with the upper limit usually being 360 minutes or less. Therefore, the ozone treatment time is usually about 1 to 360 minutes, and preferably about 30 to 360 minutes. When the ozone treatment conditions are within the above-mentioned ranges, it is possible to prevent cellulose from being excessively oxidized and decomposed, and the yield of oxidized cellulose can be improved.

The ozone-treated cellulose may be further subjected to a post-oxidation treatment using an oxidizing agent. The oxidizing agent used in the post-oxidation treatment is not particularly limited, but examples include chlorine compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. Examples of the method of post-oxidation treatment include a method in which an oxidizing agent is dissolved in a polar organic solvent such as water or alcohol to prepare an oxidizing agent solution, and the cellulose-based raw material is immersed in the oxidizing agent solution. The amount of carboxyl groups, carboxylate groups, and aldehyde groups contained in the oxidized cellulose can be adjusted by controlling the oxidation conditions, such as the amount of oxidizing agent added and the reaction time.

(Salt and acid forms)
When cellulose is oxidized, the hydroxyl groups of the cellulose are modified to carboxyl groups, and the cellulose fibers after the oxidation usually contain both a group represented by -COOH (an acid-type carboxyl group) and a group represented by ^-COO- (a salt-type carboxyl group). The counter cation of the salt-type carboxyl group is not particularly limited, and examples thereof include alkali metal ions such as sodium ions and potassium ions, and other metal ions.

The amount of carboxyl groups in oxidized cellulose is preferably 0.1 mmol/g or more, 0.3 mmol/g or more, or 0.5 mmol/g or more, more preferably 0.6 mmol/g or more, and even more preferably 1.0 mmol/g or more, based on the bone dry mass. The upper limit is preferably 2.5 mmol/g or less, more preferably 2.0 mmol/g or less, or 1.5 mmol/g or less. Therefore, 0.1 to 2.5 mmol/g, 0.3 to 2.5 mmol/g, or 0.5 to 2.5 mmol/g are preferred, 0.6 to 2.5 mmol/g is more preferred, and 1.0 to 2.0 mmol/g is even more preferred. The amount of carboxyl groups in oxidized cellulose microfibrils is usually the same as that of oxidized cellulose before fibrillation. The amount of carboxyl groups can be calculated from the change in electrical conductivity.

The degree of carboxy substitution per anhydrous glucose unit of oxidized cellulose is preferably 0.01 or more, 0.02 or more, or 0.05 or more, more preferably 0.10 or more, even more preferably 0.15 or more, even more preferably 0.20 or more, and particularly preferably 0.25 or more. This ensures a degree of substitution that can obtain the effects of chemical modification. The upper limit of the degree of substitution is preferably 0.50 or less, more preferably 0.40 or less, or 0.35 or less. This makes it difficult for the cellulose fibers to dissolve in water, and allows the fiber form to be maintained in water. Therefore, the degree of carboxy substitution is preferably 0.01 to 0.50, more preferably 0.05 to 0.40, and even more preferably 0.10 to 0.35. The degree of carboxy substitution of oxidized cellulose microfibrils is usually the same as that of oxidized cellulose before fibrillation. The degree of carboxy substitution can be adjusted by the reaction conditions. The degree of carboxy substitution per anhydroglucose unit is the ratio of hydroxyl groups originally possessed by each anhydroglucose (glucose residue) constituting cellulose that have been substituted with carboxyl groups (the number of carboxyl groups per glucose residue). The degree of carboxy substitution can be calculated from the amount of carboxyl groups.

(Acid-form oxidized cellulose and desalting)
Oxidized cellulose contains carboxyl groups as a result of oxidation, but may contain more acid-type carboxyl groups than salt-type carboxyl groups, or may contain more salt-type carboxyl groups than acid-type carboxyl groups. Oxidized cellulose may further be subjected to a desalting treatment, which allows the salt-type carboxyl groups to be converted into acid-type carboxyl groups. In this specification, the term "acid type" indicates that the cellulose has been desalted, and the term "salt type" indicates that the cellulose has not been desalted. The proportion of acid-type carboxyl groups among the carboxyl groups in acid-type cellulose is preferably 40% or more, more preferably 60% or more, and even more preferably 85% or more. The proportion of acid-type carboxyl groups can be calculated by the following procedure.
1) First, 250 mL of an aqueous dispersion of acid-type oxidized cellulose before desalting treatment is prepared, with a solids concentration of 0.1% by mass. A 0.1 M aqueous hydrochloric acid solution is added to the prepared aqueous dispersion to adjust the pH to 2.5, and then a 0.1 N aqueous sodium hydroxide solution is added and the electrical conductivity is measured until the pH reaches 11. The amount of acid-type carboxyl groups and the amount of salt-type carboxyl groups, i.e., the total amount of carboxyl groups, is calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of a weak acid, where the change in electrical conductivity is gradual:
Total amount of carboxyl groups (mmol/g oxidized cellulose (salt type))=a (ml)×0.1/mass of oxidized cellulose (salt type) (g)
2) 250 mL of an aqueous dispersion of desalted acid-form oxidized cellulose with a solids concentration of 0.1% by mass is prepared. 0.1 N aqueous sodium hydroxide solution is added to the prepared aqueous dispersion, and the electrical conductivity is measured until the pH reaches 11. The amount of acid-form carboxyl groups is calculated from the amount of sodium hydroxide (b) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity is gradual, using the following formula:
Amount of acid-type carboxyl groups (mmol/g acid-type oxidized cellulose)=b (ml)×0.1/mass of acid-type oxidized cellulose (g)
3) The proportion of acid-type carboxyl groups is calculated from the calculated total amount of carboxyl groups and the amount of acid-type carboxyl groups using the following formula.
Proportion of acid type carboxyl groups (%)=(amount of acid type carboxyl groups/total amount of carboxyl groups)×100

The timing of desalting may be after oxidation, and may be either before or after defibration (before or after step (2)), but is usually after oxidation, and preferably before step (2). Desalting is usually carried out by substituting salts (e.g., sodium salts) contained in the salt-type oxidized cellulose with protons. Examples of desalting methods include a method of adjusting the system to be acidic, and a method of contacting oxidized cellulose with a cation exchange resin. In the method of adjusting the system to be acidic, the pH of the system is preferably adjusted to 2 to 6, more preferably 2 to 5, and even more preferably 2.3 to 5. To adjust the system to be acidic, an acid (e.g., inorganic acids such as sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, nitrous acid, and phosphoric acid; organic acids such as acetic acid, lactic acid, oxalic acid, citric acid, and formic acid) is usually used. After the addition of the acid, a washing treatment may be appropriately carried out. As the cation exchange resin, either a strongly acidic ion exchange resin or a weakly acidic ion exchange resin can be used as long as the counter ion is H ⁺ . The ratio of the oxidized cellulose and the cation exchange resin when they are contacted is not particularly limited, and a person skilled in the art can set it appropriately from the viewpoint of efficient proton replacement. The cation exchange resin after contact can be recovered by a conventional method such as suction filtration.

(Etherification (e.g., carboxyalkylation))
Examples of the etherification include etherification by a reaction selected from carboxyalkylation, methylation, ethylation, cyanoethylation, hydroxyethylation, hydroxypropylation, ethylhydroxyethylation, and hydroxypropylmethylation, with carboxyalkylation being preferred and carboxymethylation being more preferred.

The modified cellulose (carboxyalkylated cellulose) obtained through carboxyalkylation preferably has a structure in which at least one of the hydroxyl groups of cellulose is carboxyalkylated. The degree of carboxyalkyl substitution (DS, preferably the degree of carboxymethyl substitution) per anhydrous glucose unit of carboxyalkylated cellulose is preferably 0.01 or more, 0.02 or more, or 0.05 or more, more preferably 0.10 or more, even more preferably 0.15 or more, even more preferably 0.20 or more, and particularly preferably 0.25 or more. This ensures a degree of substitution to obtain the effect of chemical modification. The upper limit of the degree of substitution is preferably 0.50 or less, more preferably 0.40 or less, or 0.35 or less. This makes it difficult for the cellulose fiber to dissolve in water, and the fiber form can be maintained in water. Therefore, the degree of carboxyalkyl substitution is preferably 0.01 to 0.50, more preferably 0.05 to 0.40, and even more preferably 0.10 to 0.35 or 0.20 to 0.35. The degree of carboxyalkyl substitution of the carboxyalkylated cellulose microfibrils is usually the same as that of the carboxyalkylated cellulose before fibrillation. The degree of carboxyalkyl substitution can be adjusted by controlling any one of the amount of the carboxyalkylating agent to be reacted, the amount of the mercerizing agent, and the composition ratio of water and the organic solvent.

The degree of carboxyalkyl substitution (also called the degree of etherification) per anhydroglucose unit is the proportion of hydroxyl groups originally contained in each anhydroglucose (glucose residue) constituting cellulose that have been substituted with carboxyalkyl ether groups (the number of carboxyalkyl ether groups per glucose residue). The degree of carboxyalkyl substitution can be calculated from the amount of carboxyalkyl groups.

The amount of carboxyl groups in carboxyalkylated cellulose is preferably 0.1 mmol/g or more, more preferably 0.6 mmol/g or more, and even more preferably 1.0 mmol/g or more, based on the bone dry mass. The upper limit is preferably 2.5 mmol/g or less, more preferably 2.0 mmol/g or less. Therefore, 0.1 to 2.5 mmol/g is preferable, 0.6 to 2.5 mmol/g is more preferable, and 1.0 to 2.0 mmol/g is even more preferable. The amount of carboxyl groups in carboxyalkylated cellulose microfibrils is usually the same as that of carboxyalkylated cellulose before fibrillation. The amount of carboxyl groups can be calculated from the change in electrical conductivity.

An example of a carboxyalkylation method is to mercerize the cellulosic starting material and then etherify it. Carboxymethylation is used as an example below.

Mercerization is usually carried out by mixing a cellulosic raw material, a solvent and a mercerizing agent. An example of the reaction conditions is as follows: The reaction temperature is usually 0°C or higher, preferably 10°C or higher, and the upper limit is usually 70°C or lower, preferably 60°C or lower. Therefore, the reaction temperature is usually 0 to 70°C, preferably 10 to 60°C. The reaction time is usually 15 minutes or more, preferably 30 minutes or more. The upper limit of the time is usually 8 hours or less, preferably 7 hours or less. Therefore, the reaction time is usually 15 minutes to 8 hours, preferably 30 minutes to 7 hours.

Examples of the solvent include water, organic solvents (e.g., alcohols (e.g., lower alcohols), ketones, dioxane, diethyl ether, benzene, dichloromethane), and mixed solvents thereof. Examples of the lower alcohol include methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butyl alcohol, isobutyl alcohol, and tertiary butyl alcohol. Of these, monohydric alcohols having 1 to 4 carbon atoms are preferred, and monohydric alcohols having 1 to 3 carbon atoms are more preferred, because they have good compatibility with water. Examples of the ketone include acetone, diethyl ketone, and methyl ethyl ketone. The mixing ratio of the lower alcohol in the mixed solvent is preferably 60 to 95% by mass. The amount of the solvent is usually 3 times or more by mass relative to the cellulose-based raw material. The upper limit of the amount is not particularly limited, but is usually 20 times or less by mass. Therefore, the amount of the solvent is preferably 3 to 20 times by mass.

Examples of mercerizing agents include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. The amount of mercerizing agent used is preferably 0.5 times the molar amount per anhydrous glucose residue of the starting material, more preferably 1.0 times the molar amount, and even more preferably 1.5 times the molar amount. The upper limit of the amount is usually 20 times the molar amount, preferably 10 times the molar amount, and even more preferably 5 times the molar amount. Therefore, the amount of mercerizing agent used is preferably 0.5 to 20 times the molar amount, more preferably 1.0 to 10 times the molar amount, and even more preferably 1.5 to 5 times the molar amount.

The etherification reaction is usually carried out by adding an etherification agent (e.g., a carboxymethylation agent) to the reaction system after mercerization. The following will explain carboxymethylation as an example. Examples of carboxymethylation agents include monochloroacetic acid, sodium monochloroacetate, methyl monochloroacetate, ethyl monochloroacetate, and isopropyl monochloroacetate, with monochloroacetic acid and sodium monochloroacetate being preferred. The amount of the carboxymethylation agent used is usually 0.05 or more moles, or 0.5 or more moles, preferably 0.6 or more moles, and more preferably 0.7 or more moles or 0.8 or more moles per anhydrous glucose residue of cellulose contained in the cellulosic raw material. The upper limit of the amount is usually 10.0 or less moles, 5 or less moles, 3 or less moles, or 1.5 or less moles, preferably 1.3 or less moles, and more preferably 1.1 or less moles, and therefore the amount is preferably 0.05 to 10.0 moles, more preferably 0.5 to 5 moles, and even more preferably 0.8 to 3 moles. The carboxymethylating agent can be added to the reaction system as it is or as an aqueous solution. The concentration of the aqueous solution of the carboxymethylating agent is usually 5 to 80% by mass, more preferably 30 to 60% by mass.

The reaction temperature for etherification is usually 30° C. or higher, preferably 40° C. or higher, and the upper limit is usually 90° C. or lower, preferably 80° C. or lower. Therefore, the reaction temperature is usually 30 to 90° C., preferably 40 to 80° C. The reaction time is usually 30 minutes or more, preferably 1 hour or more, and the upper limit is usually 10 hours or less, preferably 4 hours or less. Therefore, the reaction time is usually 30 minutes to 10 hours, preferably 1 hour to 4 hours. The reaction liquid may be stirred as necessary during the carboxymethylation reaction.
The molar ratio of the mercerizing agent to the carboxymethylating agent (mercerizing agent/carboxymethylating agent) is preferably 0.90 or more when monochloroacetic acid or sodium monochloroacetate is used as the carboxymethylating agent. This allows the carboxymethylation reaction to proceed sufficiently, and the remaining unreacted monochloroacetic acid or sodium monochloroacetate can be suppressed. The upper limit is preferably 2.45 or less. This prevents the mercerizing agent from becoming excessive, suppresses the progress of side reactions between the mercerizing agent and monochloroacetic acid or sodium monochloroacetate, suppresses the production of glycolic acid alkali metal salts, and allows the reaction to proceed economically. Therefore, the molar ratio of the mercerizing agent to the carboxymethylating agent is generally 0.90 to 2.45.

Examples of methods for carboxymethylation include (Method 1) the aqueous medium method (a method in which both mercerization and carboxymethylation are carried out in a solvent mainly composed of water), (Method 2) the solvent method (a method in which both mercerization and carboxymethylation are carried out in a mixed solvent of water and an organic solvent), and (Method 3) a method in which a solvent mainly composed of water is used for mercerization and a mixed solvent of an organic solvent and water is used for carboxymethylation, with Method 3 being preferred. This allows carboxymethylated cellulose to be economically obtained with a crystallinity of 50% or more, carboxymethyl groups to be introduced uniformly rather than locally while maintaining the effective utilization rate of the carboxymethylating agent, and a small absolute value of the degree of anionization.

The water content in the water-based solvent is usually more than 50% by mass, preferably 55% by mass or more, 60% by mass or more, 70% by mass or more, or 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 100% by mass. This allows the carboxymethyl group to be introduced more uniformly into the cellulose. The water-based solvent may contain a solvent other than water (e.g., the organic solvent described above). The amount of the solvent other than water is usually 45% by mass or less, preferably 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less, more preferably 5% by mass or less, and even more preferably 0% by mass. The solvent may be prepared by preparing a predetermined amount of water and, if necessary, a solvent other than water, and mixing them.

The water content of the solvents used in the mercerization reaction and the carboxymethylation reaction is preferably less in the latter (the organic solvent content is greater in the latter) (not including the water content of cellulose). This makes it easier to maintain the crystallinity of the resulting carboxymethylated cellulose, and the desired modified cellulose microfibrils can be obtained efficiently. Therefore, it is preferable to add an organic solvent or an aqueous solution of an organic solvent to the reaction system between the end of the mercerization reaction and immediately after the addition of the carboxymethylating agent, or to reduce the amount of solvent other than water during the mercerization treatment (e.g., reduced pressure treatment) to form a mixed solvent of water and an organic solvent. Of these, the former is more preferable. This allows the carboxymethylation reaction to proceed efficiently by simple means. The timing of adding or reducing the organic solvent is preferably within 30 minutes before or after the addition of the carboxymethylating agent.

The ratio of the organic solvent in the mixed solvent for carboxymethylation is preferably 20% by mass or more or 30% by mass or more, more preferably 40% by mass or more, even more preferably 45% by mass or more, and even more preferably 50% by mass or more, based on the total of water and organic solvent. This makes it easier for uniform carboxymethyl group replacement to occur, and carboxymethyl cellulose with stable quality can be obtained. The upper limit is usually 99% by mass or less, and considering the cost of the organic solvent, it is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less, and even more preferably 70% by mass or less.

The effective utilization rate (AM) of the carboxymethylating agent is preferably 15% or more, more preferably 20% or more, even more preferably 25% or more, and even more preferably 30% or more. The upper limit is not particularly limited and is substantially 80% or less. The effective utilization rate of the carboxymethylating agent means the ratio of the amount of carboxymethyl groups introduced into cellulose to the amount of carboxymethyl groups possessed by the carboxymethylating agent, and can be calculated by the following formula:
AM = (DS x moles of cellulose) / moles of carboxymethylating agent DS: degree of carboxymethyl substitution Moles of cellulose = mass of pulp / 162
The pulp mass means the dry mass when dried at 100° C. for 60 minutes, and 162 means the molecular weight per glucose unit of cellulose.

(Acid-type carboxyalkylated cellulose)
The carboxyalkylated cellulose may contain more acid type carboxyl groups than salt type carboxyl groups, or may contain more salt type carboxyl groups than acid type carboxyl groups. The carboxyalkylated cellulose may further be subjected to a desalting treatment. This allows the salt type carboxyl groups to be converted into acid type carboxyl groups. The ratio of the amount of acid type carboxyl groups to the amount of carboxyl groups in the acid type carboxyalkylated cellulose is preferably 40% or more, more preferably 60% or more, and even more preferably 85% or more. The method for calculating the ratio of acid type carboxyl groups is as described above.

The timing of desalting is usually after carboxyalkylation, preferably after etherification and before fibrillation. For example, the desalting method may be a method of contacting carboxyalkylated cellulose with a cation exchange resin. As the cation exchange resin, either a strong acid ion exchange resin or a weak acid ion exchange resin can be used as long as the counter ion is H ⁺ . The ratio of the two when contacting carboxyalkylated cellulose with a cation exchange resin is not particularly limited, and a person skilled in the art can appropriately set it from the viewpoint of efficient proton replacement. For example, the ratio can be adjusted so that the pH of the aqueous dispersion after addition of the cation exchange resin is preferably 2 to 6, more preferably 2 to 5, relative to the carboxyalkylated cellulose aqueous dispersion. The cation exchange resin after contact may be recovered by a conventional method such as suction filtration.

(Esterification (e.g., phosphate esterification))
As an example of the esterification, there is a method (phosphorylation) in which a compound having a phosphoric acid group is reacted with a cellulosic raw material. Examples of the phosphoric acid esterification method include a method in which a powder or an aqueous solution of a compound having a phosphoric acid group is mixed with the cellulosic raw material, and a method in which an aqueous solution of a compound having a phosphoric acid group is added to an aqueous dispersion of the cellulosic raw material, with the latter being preferred. This can increase the uniformity of the reaction and the efficiency of esterification.

Examples of compounds having a phosphate group include phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, esters thereof, and salts thereof. More specifically, examples include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, and ammonium metaphosphate. These are low cost and easy to handle, and can be used to introduce phosphate groups into cellulose to improve defibration efficiency. The compound having a phosphate group may be one type or a combination of two or more types. Among these, from the viewpoints of high efficiency of introduction of phosphate groups, easy fibrillation, and ease of industrial application, phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are preferred, sodium salts of phosphoric acid are more preferred, and sodium dihydrogen phosphate and disodium hydrogen phosphate are even more preferred. It is preferable to use an aqueous solution of a compound having a phosphate group in the esterification because it increases the uniformity of the reaction and increases the efficiency of introduction of phosphate groups. The pH of the aqueous solution of the compound having a phosphate group is preferably 7 or less since this increases the efficiency of introducing the phosphate group, and more preferably 3 to 7 since this also suppresses hydrolysis of the fibers.

An example of the phosphate esterification method is described below. A compound having a phosphate group is added to a suspension of a cellulose-based raw material (e.g., solid content concentration 0.1 to 10% by mass) while stirring, and phosphate groups are introduced into the cellulose. When the cellulose-based raw material is 100 parts by mass, the amount of the compound having a phosphate group added is preferably 0.2 parts by mass or more, more preferably 1 part by mass or more, in terms of the amount of phosphorus atoms. This can further improve the yield of esterified cellulose. The upper limit is preferably 500 parts by mass or less, more preferably 400 parts by mass or less. This can efficiently obtain a yield that corresponds to the amount of the compound having a phosphate group used. Therefore, 0.2 to 500 parts by mass is preferable, and 1 to 400 parts by mass is more preferable.

When reacting a compound having a phosphate group with a cellulosic raw material, a basic compound may also be added to the reaction system. Examples of methods for adding a basic compound to the reaction system include adding a basic compound to an aqueous dispersion of a cellulosic raw material, an aqueous solution of a compound having a phosphate group, or an aqueous dispersion of a cellulosic raw material and a compound having a phosphate group.

The basic compound is not particularly limited, but preferably exhibits basicity, and more preferably is a nitrogen-containing compound exhibiting basicity. "Exhibits basicity" usually means that an aqueous solution of the basic compound exhibits a pink to red color in the presence of a phenolphthalein indicator, and/or that the pH of the aqueous solution of the basic compound is greater than 7. The basic compound is preferably a compound having a nitrogen atom exhibiting basicity, and more preferably a compound having an amino group exhibiting basicity. Examples of compounds having an amino group exhibiting basicity include urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, urea is preferred because of its low cost and ease of handling. The amount of the basic compound added is preferably 2 to 1,000 parts by mass, more preferably 100 to 700 parts by mass. The reaction temperature is preferably 0 to 95°C, more preferably 30 to 90°C. The reaction time is not particularly limited, but is usually about 1 to 600 minutes, and preferably 30 to 480 minutes. When the esterification reaction conditions are within any of these ranges, the cellulose can be prevented from being excessively esterified and becoming easily soluble, and the yield of phosphated cellulose can be improved.

After reacting the cellulosic raw material with a compound having a phosphate group, a suspension of cellulose phosphate is obtained. The suspension of cellulose phosphate may be dehydrated as necessary. It is preferable to carry out a heat treatment after dehydration, which can suppress hydrolysis of cellulose. The heating temperature is preferably 100 to 170°C. It is more preferable to heat the material at 130°C or less (preferably 110°C or less) while it contains water during the heat treatment, and then heat the material at 100 to 170°C after removing the water.

The degree of phosphate group substitution per glucose unit of the phosphated cellulose is preferably 0.001 or more. This allows sufficient fibrillation to be performed. The upper limit is preferably less than 0.40. This allows the swelling or dissolution of the phosphated cellulose to be suppressed. Therefore, the degree of phosphate group substitution is preferably 0.001 or more and less than 0.40. By the phosphate esterification reaction, phosphate group substituents are introduced into the cellulose, and the cellulose particles repel each other electrically. Therefore, the phosphated cellulose can be easily fibrillated. After the phosphate esterification, it is preferable to perform a washing treatment such as boiling and washing with cold water. This allows efficient fibrillation.

<Step (2): Defibrillation (fibrillation) step>
In step (2), the modified cellulose obtained in step (1) is mechanically treated to defibrate the cellulose at a pH of 7 or less and a solid content of 0.1 to 15% by mass. This produces modified cellulose microfibrils. Defibration (fibrillation) is usually achieved by a mechanical treatment, which is preferably a defibration or beating treatment. The mechanical treatment (preferably a beating or defibration treatment) is usually carried out in a wet manner (i.e., in the form of an aqueous dispersion). Examples of the apparatus used for the mechanical treatment include refiners (e.g., disk type, conical type, cylinder type), high-speed defibrators, shear type agitators, colloid mills, high-pressure jet dispersers, beaters, PFI mills, kneaders, dispersers, high-speed disintegrators (top finers), high-pressure or ultra-high-pressure homogenizers, grinders (stone-type grinders), ball mills, vibration mills, bead mills, single-axis, twin-axis, or multi-axis kneaders/extruders, homomixers under high-speed rotation, refiners, defibrators, friction grinders, high-shear defibrators, etc. Examples of the device include a device capable of imparting a mechanical defibration force, such as a defibrator, a disperger, or a homogenizer (e.g., a microfluidizer), of which a device capable of imparting a wet defibration force is preferred, and a high-speed disintegrator or a refining device is more preferred, but there are no particular limitations on the device.

When defibration is performed in a wet manner, the modified cellulose obtained in step (1) is usually dispersed in water to prepare an aqueous dispersion, and the aqueous dispersion is subjected to mechanical treatment. The solid content concentration of the modified cellulose in the aqueous dispersion obtained in step (1) is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more. The upper limit of the concentration is preferably 15% by mass or less, more preferably 10% by mass or less, and even more preferably 8% by mass or less. The pH during mechanical treatment is preferably 7 or less, more preferably 6 or less, and even more preferably 5 or less. In the past, from the viewpoint of improving defibration, a pH adjuster such as an alkali was added before the mechanical treatment to adjust the pH to more than 7, but in the present invention, it has been found that the desired microfibrils can be obtained by setting the pH to 7 or less. The lower limit of the pH is not particularly limited, but is usually 2 or more, preferably 3 or more, and more preferably 3.5 or more. The pH can be adjusted by adding a pH adjuster such as an acid (e.g., hydrochloric acid) or an alkali (e.g., sodium hydroxide, sodium bicarbonate) to the aqueous dispersion. Prior to the preparation of the aqueous dispersion, the chemically modified cellulose may be subjected to a pretreatment such as dry grinding (e.g., grinding after drying). Examples of the apparatus used for dry grinding include, but are not limited to, impact mills such as hammer mills and pin mills, media mills such as ball mills and tower mills, and jet mills. Treatments for adjusting the pH and solid content concentration may be appropriately performed after the end of step (1), but if the pH and solid content concentration are already within the above ranges at the end of step (1) (preferably during the washing treatment performed after modification as necessary), the adjustments can be omitted.

The defibration conditions can be appropriately selected so that the average fiber diameter, average fiber length, and aspect ratio after treatment are within the ranges described below. As a result, the resulting modified cellulose microfibrils can exhibit higher water retention than undefibrated cellulose fibers. Compared to finely defibrated cellulose nanofibers, even a small amount can exhibit high strength-improving effects and yield-improving effects. The defibration conditions can be appropriately selected so that the fibrillation rate or the difference Δf in the fibrillation rates of the samples before and after defibration is within the above-mentioned range. At the end of defibration, it is preferable to adjust the pH of the aqueous dispersion to near neutral (about 7.0, for example 6.5 to 7.5) as necessary. The pH can be adjusted, for example, by adding the above-mentioned pH adjuster to the aqueous dispersion.

<Optional post-processing>
The modified cellulose microfibrils may be used as a final product in the form of an aqueous dispersion obtained after the completion of step (2), or may be subjected to post-treatment as necessary. Examples of the post-treatment method include, but are not limited to, drying (e.g., freeze drying, spray drying, tray drying, drum drying, belt drying, thin spreading on a glass plate or the like and drying, fluidized bed drying, microwave drying, and heated fan-type reduced pressure drying), redispersion in water (dispersion apparatus is not limited), and pulverization (e.g., pulverization using equipment such as a cutter mill, hammer mill, pin mill, or jet mill).

<Modified cellulose microfibrils>
In the present invention, the modified cellulose microfibril is usually a fibrillated fiber of modified cellulose. By undergoing step (2), the specific surface area is increased compared to before the treatment, and it is expected that the water retention and strength imparting effects will be improved. In addition, by undergoing step (1), the fibers are easily loosened during step (2), and fibrillation can be efficiently carried out with less power compared to when no modification is performed. In addition, affinity with water is improved, and good water retention can be exhibited even if the fiber length is relatively long.

(Shape of modified cellulose microfibrils)
The characteristics of the shape of the modified cellulose microfibrils are as follows. Compared with modified cellulose that has not undergone fibrillation, fluffing of cellulose microfibrils is usually observed on the fiber surface. Compared with chemically modified cellulose nanofibers, fineness of the fiber itself is usually suppressed, and fluffing (external fibrillation) on the fiber surface is efficiently achieved. Compared with fibrillated cellulose nanofibers that have not been chemically modified, water retention is good, and thixotropy is observed. The modified cellulose microfibrils are preferably fibrillated fibers of chemically modified cellulose. This allows the fibers to easily come apart during fibrillation, suppressing damage to the fibers.

(Average fiber diameter, average fiber length, fiber length distribution and aspect ratio)
The average fiber diameter of the modified cellulose microfibrils is usually 500 nm or more, preferably 1 μm or more, and more preferably 10 μm or more. This allows the microfibrillated cellulose fibers to exhibit higher water retention than undefibrated cellulose fibers, and even a small amount of the microfibrillated cellulose fibers can provide higher strength and yield improvement effects than finely defibrated cellulose nanofibers. The upper limit of the average fiber diameter is preferably 60 μm or less, more preferably 40 μm or less, even more preferably 30 μm or less, and even more preferably 20 μm or less, 18 μm or less, or 17 μm or less, but there is no particular limit.

The average fiber length is preferably 500 μm or more or 550 μm or more, more preferably 600 μm or more, 700 μm or more, or 800 μm or more. The upper limit of the average fiber length is not particularly limited, but is preferably 3,000 μm or less, preferably 2,500 μm or less, more preferably 2,000 μm or less, and even more preferably 1,500 μm or less.

The fiber length distribution preferably satisfies any one of the following, and more preferably the fiber length of 0.6 to 1.2 mm satisfies the following range:
The proportion of fiber lengths of 0 to 0.2 mm is preferably 50% or less, more preferably 45% or less (the lower limit is preferably 3% or more, more preferably 5% or more).
The proportion of fiber lengths of 0.2 to 0.6 mm is preferably 50% or less, more preferably 45% or less (the lower limit is preferably 10% or more, more preferably 15% or more).
The proportion of fiber lengths of 0.6 to 1.2 mm is preferably 18% or more, more preferably 20% or more (the upper limit is preferably 45% or less, more preferably 40% or less).
The proportion of fiber lengths of 1.2 to 2.0 mm is preferably 1% or more, more preferably 1.05% or more (the upper limit is preferably 50% or less, more preferably 45% or less).
The proportion of fiber lengths between 2.0 and 3.2 mm is preferably 0.02% or more, more preferably 0.05% or more (the upper limit is preferably 20% or less, more preferably 15% or less).
The proportion of fiber lengths between 3.2 and 7.6 mm is preferably 1.5% or less, more preferably 1% or less (the lower limit may be 0 or more).

The fiber length distribution of the oxidized cellulose microfibrils preferably satisfies any one of the following:
The proportion of fiber lengths of 0 to 0.2 mm is preferably 30% or less, more preferably 25% or less (the lower limit is preferably 3% or more, more preferably 5% or more).
The proportion of fiber lengths of 0.2 to 0.6 mm is preferably 50% or less, more preferably 45% or less (the lower limit is preferably 10% or more, more preferably 15% or more).
The proportion of fiber lengths of 0.6 to 1.2 mm is preferably 20% or more, more preferably 25% or more (the upper limit is preferably 45% or less, more preferably 40% or less).
The proportion of fiber lengths of 1.2 to 2.0 mm is preferably 3% or more, more preferably 5% or more (the upper limit is preferably 50% or less, more preferably 45% or less).
The proportion of fiber lengths of 2.0 to 3.2 mm is preferably 1% or more, more preferably 2% or more (the upper limit is preferably 20% or less, more preferably 15% or less).
The proportion of fiber lengths between 3.2 and 7.6 mm is preferably 1.5% or less, more preferably 1% or less (the lower limit may be 0 or more).

The fiber length distribution of the carboxyalkylated cellulose microfibrils preferably satisfies any one of the following:
The proportion of fiber lengths of 0 to 0.2 mm is preferably 50% or less, more preferably 45% or less (the lower limit is preferably 20% or more, more preferably 30% or more).
The proportion of fiber lengths of 0.2 to 0.6 mm is preferably 50% or less, more preferably 45% or less (the lower limit is preferably 10% or more, more preferably 20% or more).
The proportion of fiber lengths of 0.6 to 1.2 mm is preferably 15% or more, more preferably 20% or more (the upper limit is preferably 30% or less, more preferably 25% or less).
The proportion of fiber lengths between 1.2 and 2.0 mm is preferably 1% or more, more preferably 1.05% or more (the upper limit is preferably 5% or less, more preferably 3% or less).
The proportion of fiber lengths between 2.0 and 3.2 mm is preferably 0.01% or more, more preferably 0.04% or more (the upper limit is preferably 5% or less, more preferably 3% or less).
The proportion of fiber lengths between 3.2 and 7.6 mm is preferably 1.5% or less, more preferably 1% or less (the lower limit may be 0 or more).

The average fiber diameter, average fiber length, and fiber length distribution can be determined using a fractionator (e.g., a fractionator manufactured by Valmet Co., Ltd.). When a fractionator is used, the length-weighted fiber width, length-weighted average fiber length, and f1 to f6(l)%: Fraction 1 to 6 percentage of length weighted distribution can be determined, respectively.

The aspect ratio of the modified cellulose microfibrils is preferably 30 or more or 35 or more, more preferably 40 or more, even more preferably 50 or more, and even more preferably 60 or more. There is no particular upper limit to the aspect ratio, but it is preferably 1000 or less, more preferably 100 or less, and even more preferably 80 or less. The aspect ratio can be calculated by the following formula:
Aspect ratio = average fiber length/average fiber diameter.

(Transparency)
The transparency (transmittance of 660 nm light) of the aqueous dispersion of modified cellulose microfibrils (solid content concentration of 1% by mass) is preferably less than 60%, more preferably 40% or less, even more preferably 30% or less, even more preferably 20% or less, and particularly preferably 13% or less. This ensures a moderate degree of fibrillation, and allows the effects of the present invention to be fully obtained. The lower limit may be 0% or more, and is not particularly limited. The aqueous dispersion of modified cellulose microfibrils (solid content concentration of about 2% or more) is usually translucent to white, and is in the form of a gel, cream, or paste.

In this specification, unless otherwise specified, the aqueous dispersion of modified cellulose microfibrils means a dispersion obtained by dispersing the above-mentioned fibers in water as a dispersion medium.

(Specific surface area)
The BET specific surface area of the modified cellulose microfibrils is preferably 30 ^m2 /g or more, more preferably 40 ^m2 /g or more, and even more preferably 50 ^m2 /g or more. The upper limit is preferably 200 ^m2 /g or less, more preferably 150 ^m2 /g or less, and even more preferably 100 ^m2 /g or less. The BET specific surface area can be measured by substituting t-BuOH in the aqueous dispersion and then freeze-drying the sample according to the nitrogen gas adsorption method (JIS Z 8830) using a BET specific surface area meter.

(Crystallization degree of cellulose type I)
The crystallinity of cellulose type I in the modified cellulose microfibril is usually 50% or more, preferably 60% or more, 70% or more, or 75% or more, more preferably 80% or more. The upper limit is not particularly limited, but in reality it is considered to be about 90%. The crystallinity of cellulose can be controlled by the degree of chemical modification. The crystallinity of cellulose type I can be calculated by measuring and comparing the intensity of the (200) peak around 22.6° and the valley between (200) and (110) (around 18.5°) by X-ray diffraction measurement.

(Degree of Anionization)
The degree of anionization (anion charge density) of the modified cellulose microfibril is usually 2.50 meq/g or less, preferably 2.30 meq/g or less, more preferably 2.0 meq/g or less, and even more preferably 1.50 meq/g or less, 1.0 meq/g or less, 0.8 meq/g or less, or 0.5 meq/g or less. This is thought to make the chemical modification more uniform throughout the cellulose than chemically modified cellulose fibers having a higher degree of anionization, and it is thought that the effects unique to chemically modified cellulose fibers, such as water retention, can be obtained more stably. The lower limit is usually 0.08 meq/g or more, preferably 0.10 meq/g or more, or 0.20 meq/g or more, more preferably 0.30 meq/g or more, but is not particularly limited. Therefore, 0.08 meq/g or more and 2.50 meq/g or less are preferred. The degree of anionization is the equivalent of anion per unit mass of modified cellulose microfibril, and can be calculated from the equivalent of diallyldimethylammonium chloride (DADMAC) required to neutralize the anionic groups in unit mass of modified cellulose microfibril.

(Water retention capacity)
The water retention capacity of the modified cellulose microfibrils is preferably 10 or more, more preferably 15 or more, or 18 or more, even more preferably 20 or more, and even more preferably 30 or more. The upper limit is thought to be about 200 or less in reality, but is not particularly limited. The water retention capacity corresponds to the mass of water in the sediment relative to the mass of the solid content of the fibers in the sediment, and is the ratio of the water content to the solid content in the precipitated gel, measured and calculated by centrifuging a 0.3 mass% aqueous dispersion of the fibers at 25,000 G. That is, it is calculated by the following formula:
Water retention capacity=(B+C-0.003×A)/(0.003×A-C)
A: mass of an aqueous dispersion of modified cellulose microfibrils having a solid content concentration of 0.3% by mass. B: mass of a sediment separated after centrifuging the aqueous dispersion of mass A under conditions of 30° C., 25,000 G, and 30 minutes. C: mass of solids in the aqueous phase separated after the centrifugation.

The higher the water retention value, the stronger the fiber's ability to retain water. Water retention can be measured or calculated for fibers that have been fibrillated, but cannot usually be measured for fibers that have not been fibrillated or defibrated, or for cellulose nanofibers that have been defibrated down to single microfibrils. When cellulose fibers that have not been fibrillated or defibrated are centrifuged under the above-mentioned conditions, a dense sediment cannot be formed, and it is difficult to separate the sediment from the aqueous phase. When cellulose nanofibers are centrifuged under the above-mentioned conditions, there is usually very little sedimentation.

(viscosity)
When the modified cellulose microfibril is made into an aqueous dispersion, it is preferable that the viscosity of the aqueous dispersion is low. This allows the material to have good handleability despite being fibrillated. For example, the B-type viscosity (25°C, 60 rpm) of an aqueous dispersion having a solid content of 1% by mass is usually 3,500 mPa·s or less, preferably 3,000 mPa·s or less, more preferably 2,500 mPa·s or less, even more preferably 2,000 mPa·s or less, 1,000 mPa·s or less, 800 mPa·s or less, 500 mPa·s or less, 400 mPa·s or less, or 300 mPa·s or less. The lower limit is preferably 10 mPa·s or more, more preferably 20 mPa·s or more, even more preferably 50 mPa·s or more, 70 mPa·s or more, or 100 mPa·s or more. The B-type viscosity can be measured, for example, by the following method. After fibrillation (e.g., defibration), the mixture is left to stand for at least one day, diluted as necessary, stirred with a homodisper (e.g., 3,000 rpm, 5 min), and then the viscosity is measured (e.g., the viscosity is measured after rotation at 60 rpm for 3 minutes).

(Fibrillation rate)
The fibrillation rate (Fibrillation %) of the modified cellulose microfibrils is preferably 1.0% or more, more preferably 1.2% or more, and even more preferably 1.5% or more. This allows confirmation that the fibrillation is sufficient. The fibrillation rate can be adjusted depending on the type of cellulose-based raw material used. The fibrillation rate can be determined by an image analysis type fiber analyzer such as a fractionator manufactured by Valmet Co., Ltd.

(Electrical Conductivity)
The electrical conductivity of the aqueous dispersion of modified cellulose microfibrils (solid content concentration 1.0% by mass) is preferably 500 mS/m or less, more preferably 300 mS/m or less, even more preferably 200 mS/m or less, even more preferably 100 mS/m or less, and particularly preferably 70 mS/m or less. The lower limit is preferably 5 mS/m or more, more preferably 10 mS/m or more, 13 mS/m or more, 15 mS/m or more, 18 mS/m or more, 20 mS/m or more, 30 mS/m or more, 40 mS/m or more, or 50 mS/m or more. The electrical conductivity can be measured by preparing 200 g of an aqueous dispersion of modified cellulose microfibrils having a solid content concentration of 1.0% by mass, and using an electrical conductivity meter (HORIBA ES-71 type).

(Freeness)
The Shopper-Riegler freeness of the modified cellulose microfibrils is preferably 1° SR or more, more preferably 10° SR or more, more preferably 25° SR or more, even more preferably 40° SR or more, and even more preferably 50° SR or more. The upper limit is not particularly limited as long as it is 100° SR or less. The Shopper-Riegler freeness is a measure of the degree of drainage of a suspension of fibers, with the lower limit being 0° SR and the upper limit being 100° SR. The closer to 100° SR, the less drainage (amount of drainage) is indicated. The Shopper-Riegler freeness may be measured in accordance with JIS P 8121-1:2012. An example will be described below. The fibrillated chemically modified cellulose fibers are dispersed in water to prepare an aqueous dispersion with a solid content of 10 g/L, and the mixture is stirred at 1000 rpm for 10 minutes or more using a magnetic stirrer. The obtained aqueous dispersion is diluted to 2 g/L. A 60 mesh screen (wire thickness 0.17 mm) is set on a DFR-04 manufactured by Mutec Corporation, and the amount of liquid passing through the mesh from 1000 ml of test liquid is measured for 60 seconds, and the Schopper-Riegler freeness is calculated according to a method in accordance with JIS P 8121-1:2012.

The modified cellulose microfibrils of the present invention have long fiber length, a large aspect ratio, and can exhibit a moderately low viscosity. Therefore, they can be used as thickeners, gelling agents, pastes, food additives, excipients, paint additives, adhesive additives, abrasives, plastic compounding materials, water retention materials, shape retention agents, muddy water regulators, filter aids, overflow prevention agents, admixtures, etc. in various fields such as food, beverages, cosmetics, medicines, papermaking, various chemical products, paints, sprays, agricultural chemicals, civil engineering, construction, electronic materials, flame retardants, household goods, adhesives, detergents, fragrances, lubricants, etc.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these. Note that, unless otherwise specified, parts and % indicate parts by mass and % by mass.

<Procedure for measuring physical properties of modified cellulose microfibril (hereinafter sometimes referred to as MFC)>
(Optical properties)
Transparency: A water dispersion (solid concentration 1% (w/v), dispersion medium: water) was prepared, and the transmittance of light with a wavelength of 660 nm was measured using a UV-VIS spectrophotometer UV-1800 (manufactured by Shimadzu Corporation) using a square cell with an optical path length of 10 mm (blank: ion-exchanged water).

(Chemical properties)
Carboxyl (COOH) group amount: 60 ml of a 0.5% by mass aqueous dispersion of the sample was prepared, and a 0.1 M aqueous hydrochloric acid solution was added to adjust the pH to 2.5. Then, a 0.05 N aqueous sodium hydroxide solution was added dropwise, and the electrical conductivity was measured until the pH reached 11. The amount of carboxyl (COOH) group was calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity was gradual:
Amount of carboxyl groups [mmol/g carboxylated cellulose]=a [ml]×0.05/mass of carboxylated cellulose [g].

Degree of substitution: Approximately 2.0 g of the sample was weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of nitric acid methanol (a solution of 100 mL of concentrated nitric acid added to 1000 mL of methanol) was added and shaken for 3 hours to convert the salt-type carboxyl group to the acid-type. 1.5 to 2.0 g of the obtained acid-type sample (absolutely dry) was weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. The sample was moistened with 15 mL of 80% methanol, 100 mL of 0.1 N-NaOH was added, and the mixture was shaken at room temperature for 3 hours. Excess NaOH was back-titrated with 0.1 N-H ₂ SO ₄ using phenolphthalein as an indicator, and the degree of substitution (DS value) was calculated by the following formula.
A = [(100 × F' - _0.1N - _H2SO4 (mL) × F) × 0.1] / (bone dry mass of acid-type sample (g))
Substitution degree = 0.162×A/(1-0.058×A)
F': Factor of 0.1N NaOH F: _Factor of 0.1N _H2SO4 .

Crystallinity of cellulose type I: A sample is placed in a glass cell and measured using an X-ray diffraction measuring device (e.g., LabX XRD-6000, manufactured by Shimadzu Corporation). The crystallinity is calculated using the method of Segal et al. For example, the diffraction intensity at 2θ=10° to 30° in the X-ray diffraction pattern is used as a baseline, and the crystallinity is calculated from the diffraction intensity of the 002 plane at 2θ=22.6° and the diffraction intensity of the amorphous part at 2θ=18.5° according to the following formula.
Xc=(I _002C - Ia)/I _002C ×100
Xc: Crystallinity of cellulose type I (%)
I _002C : 2θ = 22.6°, diffraction intensity of the 002 plane Ia: 2θ = 18.5°, diffraction intensity of the amorphous portion The sample for measuring the crystallinity was a freeze-dried sample prepared in the same manner as in items (1) to (9) in the measurement of the specific surface area described later, which was molded into a tablet shape and used.

- Method for measuring the degree of anionization: The modified cellulose microfibrils were dispersed in water to prepare an aqueous dispersion with a solid content of 10 g/L, and the dispersion was stirred at 1000 rpm for 10 minutes or more using a magnetic stirrer. The aqueous dispersion thus obtained was diluted to 0.1 g/L, and 10 ml of the dispersion was taken and titrated with 1/1000 normality diallyldimethylammonium chloride (DADMAC) using a streaming current detector (Mutek Particle Charge Detector 03). The amount of DADMAC added until the streaming current became zero was used to calculate the degree of anionization according to the following formula:
q = (V × c) / m
q: degree of anionization (meq/g)
V: Amount of DADMAC added until the flow current becomes zero (L)
c: DADMAC concentration (meq/L)
m: mass (g) of modified cellulose microfibrils in the measurement sample.

- Electrical conductivity: 200 g of an aqueous dispersion of the sample with a solid content concentration of 1.0% by mass was prepared and thoroughly stirred. The electrical conductivity was then measured using an electrical conductivity meter (HORIBA ES-71 model).

(Fiber properties)
Average fiber width, average fiber length, and fiber length distribution: The aqueous dispersion diluted to a solid content concentration of 0.25% by mass was subjected to a fractionator, and the length-weighted fiber width, length-weighted average fiber length, and f1 to f6(l)%: Fraction 1 to 6 percentage of length weighted distribution were obtained (n=2, Table 1).

Aspect ratio: Calculated from the measured fiber width and fiber length using the following formula.
Aspect ratio = average fiber length/average fiber diameter

- Specific surface area: (1) About 2% aqueous dispersion of modified cellulose microfibrils was separated and placed in a centrifuge container so that the solid content was about 0.1 g, and 100 ml of ethanol was added.
(2) A stirrer was added and the mixture was stirred at 500 rpm for at least 30 minutes.
(3) The stirrer was removed, and the modified cellulose microfibrils were precipitated in a centrifuge at 7,000 G for 30 minutes at 30° C.
(4) The supernatant was removed while trying to avoid removing as much of the modified cellulose microfibrils as possible.
(5) 100 ml of ethanol was added, a stirrer was added, and the mixture was stirred under the conditions of (2), centrifuged under the conditions of (3), and the supernatant was removed under the conditions of (4). This process was repeated three times.
(6) The solvent in (5) was changed from ethanol to t-butanol, and stirring, centrifugation, and removal of the supernatant were repeated three times in the same manner as in (5) at room temperature above the melting point of t-butanol.
(7) After the final removal of the solvent, 30 ml of t-butanol was added and mixed gently, then the mixture was transferred to an eggplant flask and frozen in an ice bath.
(8) Cool in a freezer for at least 30 minutes.
(9) The mixture was placed in a freeze-dryer and freeze-dried for 3 days.
(10) BET measurement was performed (pretreatment conditions: under nitrogen flow, 105° C., 2 hours, relative pressure 0.01 to 0.30, sample amount approximately 30 mg).

Water retention capacity: 40 mL of an aqueous dispersion of modified cellulose microfibrils with a solid content concentration of 0.3% by mass was prepared. The mass of the aqueous dispersion at this time was designated A. Next, the entire amount of the aqueous dispersion was centrifuged at 30°C and 25,000G for 30 minutes in a high-speed refrigerated centrifuge to separate the aqueous phase and the sediment. The mass of the sediment at this time was designated B. In addition, the aqueous phase was placed in an aluminum cup and dried overnight at 105°C to remove water, and the mass of the solid content in the aqueous phase was measured. The mass of the solid content in this aqueous phase was designated C. The water retention capacity was calculated using the following formula:
Water retention capacity=(B+C-0.003×A)/(0.003×A-C).

Fibrillation rate: Measured using a fractionator manufactured by Valmet Co., Ltd.

(viscosity)
- Type B viscosity (25°C, 60 rpm): After leaving to stand for one day or more after defibration, the viscosity was measured by one of the following methods: After diluting to a solid content of 1%, the mixture was stirred at 3000 rpm for 5 minutes using a homodisper, and then the viscosity measurement was started (60 rpm) and the viscosity value after 3 minutes was recorded.

Example 1 (TEMPO oxidation MFC (H-type, long))
<TEMPO oxidation of pulp>
5.00 kg (bone-dry) of bleached unbeaten kraft pulp (NBKP, Nippon Paper Industries Co., Ltd., brightness 85%) derived from coniferous trees was added to 500 liters of an aqueous solution in which 39 g (0.05 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 g of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. Aqueous sodium hypochlorite solution was added to the reaction system so that the sodium hypochlorite concentration was 5.5 mmol/g per 1 g of bone-dry cellulose, and the oxidation reaction was started at room temperature. The pH of the system decreased during the reaction, but 3M aqueous sodium hydroxide solution was added successively to adjust the pH to 10. The reaction was terminated when sodium hypochlorite was consumed and the pH of the system no longer changed. After the reaction, the mixture was adjusted to pH 2 by adding hydrochloric acid, and then filtered through a glass filter to separate the pulp. The separated pulp was thoroughly washed with water to obtain a TEMPO oxidized pulp. The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the amount of carboxyl groups was 1.37 mmol/g.

<Microfibrillation>
An aqueous dispersion (pH 4.5) of the obtained TEMPO oxidized pulp with a solid content concentration of 4.0% by mass was prepared and treated for 10 minutes using a B-type Topfiner (manufactured by Aikawa Iron Works Co., Ltd.). After completion of microfibrillation, a 5% NaOH aqueous solution was added to adjust the pH to 7.4 to prepare oxidized cellulose microfibrils (TEMPO oxidized MFC). The physical properties of the obtained oxidized cellulose microfibrils are shown in Table 2.

Example 2
<TEMPO oxidation of pulp>
5.00 kg (bone-dry) of bleached unbeaten kraft pulp (NBKP, Nippon Paper Industries Co., Ltd., brightness 85%) derived from coniferous trees was added to 500 liters of an aqueous solution in which 39 g (0.05 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 g of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. Aqueous sodium hypochlorite solution was added to the reaction system so that the sodium hypochlorite concentration was 2.2 mmol/g per 1 g of bone-dry cellulose, and the oxidation reaction was started at room temperature. The pH of the system decreased during the reaction, but 3M aqueous sodium hydroxide solution was added successively to adjust the pH to 10. The reaction was terminated when sodium hypochlorite was consumed and the pH of the system no longer changed. After the reaction, the mixture was adjusted to pH 2 by adding hydrochloric acid, and then filtered through a glass filter to separate the pulp. The separated pulp was thoroughly washed with water to obtain a TEMPO oxidized pulp. The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the amount of carboxyl groups was 0.56 mmol/g.

<Microfibrillation>
An aqueous dispersion (pH 4.6) of the obtained TEMPO oxidized pulp with a solid content concentration of 3.8% by mass was prepared and treated for 10 minutes using a B-type Topfiner (manufactured by Aikawa Iron Works Co., Ltd.). After completion of microfibrillation, a 5% NaOH aqueous solution was added to adjust the pH to 7.5 to prepare oxidized cellulose microfibrils (TEMPO oxidized MFC). The physical properties of the obtained oxidized cellulose microfibrils are shown in Table 2.

Example 3
<TEMPO oxidation of pulp>
5.00 kg (bone-dry) of bleached unbeaten kraft pulp (NBKP, Nippon Paper Industries Co., Ltd., brightness 85%) derived from coniferous trees was added to 500 liters of an aqueous solution in which 19.6 g (0.05 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 g of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. A sodium hypochlorite aqueous solution was added to the reaction system so that the sodium hypochlorite was 5.6 mmol/g per 1 g of bone-dry cellulose, and the oxidation reaction was started at room temperature. The pH of the system decreased during the reaction, but 3M aqueous sodium hydroxide solution was added successively to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system did not change. After the reaction, the mixture was adjusted to pH 2 by adding hydrochloric acid, and then filtered through a glass filter to separate the pulp. The separated pulp was thoroughly washed with water to obtain a TEMPO oxidized pulp. The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the amount of carboxyl groups was 1.42 mmol/g.

<Microfibrillation>
Water, 5% NaOH, and sodium bicarbonate were added to the obtained TEMPO oxidized pulp to prepare an aqueous dispersion (pH 5.5) with a solids concentration of 4.2% by mass, and the dispersion was treated for 10 minutes using a B-type Topfiner (manufactured by Aikawa Iron Works Co., Ltd.). After completion of microfibrillation, a 5% aqueous NaOH solution was further added to adjust the pH to 7.5 to prepare oxidized cellulose microfibrils (TEMPO oxidized MFC). The physical properties of the obtained oxidized cellulose microfibrils are shown in Table 2.

Example 4
The microfibrillation was carried out in the same manner as in Example 1, except that the solids concentration of the TEMPO oxidized pulp in the aqueous dispersion was changed to 2.0 mass% (Table 2).

Example 5
The microfibrillation was carried out in the same manner as in Example 1, except that the solids concentration of the TEMPO oxidized pulp in the aqueous dispersion was changed to 3.8 mass%, the pH of the aqueous dispersion was 4.6, and treatment using a 14-inch Lab Refiner (manufactured by Aikawa Iron Works Co., Ltd.) was carried out for 22 minutes instead of the Top Finer treatment (Table 2).

Example 6
<Carboxymethylation of pulp>
A twin-shaft kneader with the rotation speed adjusted to 100 rpm was charged with 130 parts of water and 20 parts of sodium hydroxide dissolved in 100 parts of water, and 100 parts of hardwood pulp (LBKP, manufactured by Nippon Paper Industries Co., Ltd.) was added in terms of dry mass when dried at 100 ° C for 60 minutes. The mixture was stirred and mixed at 30 ° C for 90 minutes to prepare a mercerized cellulose raw material. Further, 100 parts of isopropanol (IPA) and 60 parts of sodium monochloroacetate were added while stirring, and after stirring for 30 minutes, the mixture was heated to 70 ° C and allowed to undergo a carboxymethylation reaction for 90 minutes. The concentration of IPA in the reaction medium during the carboxymethylation reaction was 30%. After the reaction was completed, the mixture was neutralized with acetic acid to obtain a carboxymethylated cellulose raw material (sodium salt) with a carboxymethyl substitution degree of 0.21 and a crystallinity of cellulose I type of 72%.

<Microfibrillation>
The obtained carboxymethylated pulp was dispersed in water (pH 6.9) with a solid content of 4.0% by mass, and treated for 10 minutes using a 14-inch lab refiner (manufactured by Aikawa Iron Works Co., Ltd.) to prepare cellulose microfibrils (CM-MFC). The physical properties of the obtained cellulose microfibrils are shown in Table 2.

Comparative example 1 (TEMPO oxidized MFC (high viscosity))
In the microfibrillation, the solid content concentration of the TEMPO oxidized pulp in the aqueous dispersion was changed to 2.0% by mass, and 5% NaOH and sodium bicarbonate were added before the topfiner treatment to adjust the pH to 8.0. The same procedure as in Example 1 was repeated except that no pH adjustment treatment was carried out after completion of microfibrillation (Table 3).

Comparative example 2 (TEMPO oxidized MFC (H type, high concentration))
In the microfibrillation, the solid content of the TEMPO oxidized pulp in the water dispersion was changed to 30% by mass, and the treatment was performed twice using a 14-inch lab refiner (manufactured by Aikawa Iron Works Co., Ltd.) instead of the top refiner treatment. The procedure was the same as in Example 1, except for the above (Table 3).

Comparative Example 3
Microfibrillation was carried out in the same manner as in Example 1, except that 5% NaOH and sodium bicarbonate were added before the top-finer treatment to adjust the pH of the aqueous dispersion of TEMPO oxidized pulp to 8.6, and no pH adjustment treatment was performed after the completion of microfibrillation (Table 3).

Comparative Example 4
<TEMPO oxidation of pulp>
5.00 kg (bone-dry) of bleached unbeaten kraft pulp (NBKP, Nippon Paper Industries Co., Ltd., brightness 85%) derived from coniferous trees was added to 500 liters of an aqueous solution in which 19.6 g (0.05 mmol per 1 g of bone-dry cellulose) of TEMPO (Sigma Aldrich) and 514 g of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. A sodium hypochlorite aqueous solution was added to the reaction system so that the sodium hypochlorite was 5.9 mmol/g per 1 g of bone-dry cellulose, and the oxidation reaction was started at room temperature. The pH of the system decreased during the reaction, but 3M sodium hydroxide aqueous solution was added successively to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system did not change. After the reaction, the mixture was adjusted to pH 2 by adding hydrochloric acid, and then filtered through a glass filter to separate the pulp. The separated pulp was thoroughly washed with water to obtain a TEMPO oxidized pulp. The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the amount of carboxyl groups was 1.48 mmol/g.

<Microfibrillation>
Water, 5% NaOH, and sodium bicarbonate were added to the obtained TEMPO oxidized pulp to prepare an aqueous dispersion (pH 8.2) with a solids concentration of 4.1% by mass, and the dispersion was treated for 24 minutes using a 14-inch lab refiner (manufactured by Aikawa Iron Works Co., Ltd.) to prepare oxidized cellulose microfibrils (TEMPO oxidized MFC). The physical properties of the obtained oxidized cellulose microfibrils are shown in Table 3.

Comparative Example 5
<Carboxymethylation of pulp>
A sodium salt of carboxymethylated cellulose was obtained in the same manner as in Example 6, except that the concentration of IPA in the reaction solution during the carboxymethylation reaction was 55% by changing the amount of IPA added. The degree of carboxymethyl substitution was 0.35, and the crystallinity of cellulose type I was 68%.

<Microfibrillation>
The obtained carboxymethylated pulp was dispersed in water (pH 7.5) with a solid content of 4.0% by mass, and treated for 12 minutes using a 14-inch lab refiner (manufactured by Aikawa Iron Works Co., Ltd.) to prepare cellulose microfibrils (CM-MFC). The physical properties of the obtained cellulose microfibrils are shown in Table 3.

From the results in Tables 2 and 3, the modified cellulose microfibrils of the Examples had longer fiber lengths and larger aspect ratios than those of the Comparative Examples. In addition, the modified cellulose microfibrils of the Examples showed a moderately low viscosity. These results indicate that the modified cellulose microfibrils obtained by the present invention can exhibit excellent functionality in a wide range of applications.

Claims (5)

Step (1): A step of oxidizing a cellulosic raw material to obtain oxidized cellulose; and
Step (2): A step of defibrating the oxidized cellulose obtained in step (1) by mechanical treatment under conditions of a pH of 7 or less and a solid content concentration of 0.1 to 15% by mass;
A method for producing oxidized cellulose microfibrils , comprising:
The oxidized cellulose microfibrils have an average fiber diameter of 500 nm or more and 60 μm or less, an average fiber length of 600 μm or more and 3000 μm or less, and an aspect ratio of 50 or more . The method according to claim 1, wherein the proportion of oxidized cellulose microfibrils having fiber lengths of 2.0 to 3.2 mm is 1% or more and 20% or less. 3. The method according to claim 1 or 2 , wherein the oxidized cellulose has a carboxyl group content of 0.1 to 2.5 mmol/g. The method according to any one of claims 1 to 3 , wherein the degree of carboxyl substitution of the oxidized cellulose is from 0.01 to 0.50. The method according to any one of claims 1 to 4 , wherein the mechanical treatment is a beating or defibration treatment.